The invention pertains to a method for the production of metallic strip by the continuous casting-rolling process, in which a slab is first cast in a casting machine and then sent to a finishing mill located downstream in the transport direction, where it is rolled. The invention further pertains to a device for producing a metallic strip.
The present invention is used in casting-rolling installations which produce finished strip from molten metal in endless operation. For installations of this type, an emergency strategy is proposed for dealing with breakdowns.
Known casting-rolling lines convert molten steel to hot strip in a compact installation. The first step is to cast a slab of endless length.
These slabs are cut by shears into sections, the dimensions which correspond to the desired size of the hot coil. The slabs are conditioned to the proper temperature in heating furnaces, often designed as roller hearth furnaces. Then the slabs are sent individually to a rolling mill and rolled. After the strips have been cooled in a cooling section and wound into coils, the coils are taken from the rolling line for further processing.
In the case of the “semi-endless” method, the slab is cut in such a way that two or more coils can be produced from it. Downline from the rolling mill, a flying shear is also installed, which cuts the long hot strip to obtain a coil of the desired size. With this method, the number of critical threading-in and threading-out operations required during rolling is reduced, which makes it possible to produce thinner hot strips more reliably.
Common to both methods is that, because the slabs are cut into sections, the casting process and the rolling process can be carried out separately. The working speeds of the casting machine and the rolling mill which can and must be used can thus be determined independently of each other.
As a result of progress in the design of casting machines and in process control through the use of, for example, heating devices, it is possible today to eliminate the step of cutting the slab into sections prior to rolling. A so-called “fully continuous” process was developed. According to this process, the slab is allowed to solidify completely and is then sent to the rolling mill undivided, and all the while more molten metal is being cast onto the same strand in the casting machine. The material is not cut into coiling lengths until it reaches the flying shear downline from the rolling mill.
In this fully continuous process, therefore, operating states regularly occur in which the material forms a single physical entity extending all the way from the casting machine to the coiler. The entire process thus takes place continuously or in endless fashion.
Breakdowns occur sporadically in systems of this magnitude, which can be over several hundred meters in length. For example, when there is a malfunction in the hot strip line or a problem with the shears, etc., the production process must be interrupted. The system is then stopped and all movements of the strip or of the slab come to a standstill. It is possible for an undivided strand, the various parts of which are in different stages of processing, to extend over the entire length of the installation. Because this strand can be 100 meters or more in length as it extends through the various units (casting machine, shears, furnaces, rolling mill, coiler), a part of the strand in one unit cannot be moved independently of a part in another unit.
In principle, a breakdown can occur in any of the units; that is, it can occur in the area of the coiler, of the flying shear, of the finishing mill, of the roller hearth furnace, etc. A disruption in the finishing mill as the result of, for example, a tear in the strip between the last two stands thus leads within a very short time to a backup of material between these stands, which can be corrected only by subsequent manual operations. Several minutes of work are required for this, and then an inspection must be performed. Some repairs to parts of the installation may also be necessary.
When a breakdown occurs, the control operator or the automation system stops the rolling. The stands can usually be opened up very quickly; all of the drives are turned off; and the strand comes to a stop. Because the slab is not divided between this point and the mold, there are cases in which it is also necessary to stop the casting machine.
The casting unit is especially critical in such cases. If the shutdown lasts too long, the steel solidifies in the mold. Removing this steel is a very difficult job, which is likely to damage the mold. Opening the mold and the strand guides will usually cause the strand to rupture, allowing molten steel to pour over the unit, thus causing considerable damage. In particular, the strand guide rollers are susceptible to thermal overload during prolonged idle periods.
Removing the solidified cast strand from the casting machine is very time-consuming, and often it can be done only by cutting the strand by hand (e.g., flame cutting). Crane work is necessary here, and the mold and possibly parts of the strand casting machine must be replaced. This leads to very long down times and to lost production, and it is also associated with various types of manual operations.
EP 2 259 886 B1 proposes in this situation that the strip be cut, that the tail end of the section of strip downstream of the cut be bent upward, and that the strip upstream of the cut be cut into scrap. This method is based logically on the assumption that the upstream strip material is still moving. The method cannot be used if the strip has already come to a complete stop, that is, if the continuous process has already come to a standstill.
There is no known concept for dealing with a breakdown in which the production material has already come to a complete stop. That is, in the situation in which the production material has come to a complete stop, there is so far no way of quickly disposing of the production material by, for example, cutting it into scrap at the flying or movable shear. The previous solutions assume instead that the slab is still movable and that it is therefore possible to move the slab through a shear to chop it up. There are various types of breakdowns in which this is not possible, however, and it is especially problematic when the slab or the strip has already come to a complete stop after a production breakdown.